1. Field of the Invention
The present invention relates to a local etching method. The local etching method is to planarize or make the thickness of a semiconductor wafer uniform by locally etching projections on the surface of the semiconductor wafer with an activated species gas.
2. Description of the Related Art
FIG. 1 is a diagram for explaining the principle of a method of flattening a wafer by local dry etching using plasma. Reference numeral 100 denotes a plasma generating unit and a flow of activated species gas G contained in plasma generated by the plasma generating unit 100 is applied to the surface of a semiconductor wafer W from a nozzle 101. The semiconductor wafer W is fixed on a stage 120 which is scanned at a rate controlled in a horizontal direction relative to the nozzle 101.
The semiconductor wafer W varies in thickness according to position and has fine unevenness when it is supplied. Prior to dry etching for flattening, the thickness in each sectioned area of the semiconductor wafer W is measured. This measurement is carried out in the air to obtain data on the position of each area and the thickness of the semiconductor at that position, that is, position-thickness data. In the local dry etching method, the removal of a material in each area corresponds to a time during which the area is exposed to the activated species gas G. Therefore, the relative speed of the nozzle passing by the semiconductor wafer (to be referred to as “nozzle speed” hereinafter) is determined such that the nozzle moves relatively slowly over a relatively thick portion Wa and relatively fast over a relatively thin portion.
FIG. 2 is a graph showing the removal (depth) of the semiconductor wafer material per unit time with a flow of activated species gas, that is, etching rate distribution. This curve called “etching rate profile” is very similar to a Gaussian distribution curve. As shown in FIG. 2, the etching rate E has the maximum value Emax at the center line of the nozzle 101 and decreases as the distance from the center toward the radial direction r increases.
Thus, as the material removing capability shows a distribution according to the distance from the center of the nozzle, the removal of the material required for one area cannot be determined only by the nozzle speed of that area. That is, even if a required amount of the material is removed in one area, when an adjacent area or an area adjacent to that area is to be etched, the material in the first area is removed according to the above etching rate profile.
Thus, one area is influenced by the etching of all the other areas. Therefore, the nozzle speed is calculated so that the surface heights of all the areas become the same as a result of the totalization of these influences on all the areas.
Currently, most semiconductor wafer materials are slices of silicon monocrystals. In general, the semiconductor wafers are exposed to air before local dry etching. During this, a very thin oxide film made from SiO2 is formed on the surface. This oxide film is naturally formed but a chemically stable oxide film may be intentionally formed in a case to protect the wafer from contamination. In this case, the oxide film is generally made thicker than a natural oxide film.
When local dry etching is carried out at the nozzle speed obtained from the above position-thickness data and calculation, a desired flat surface should be obtained. However, when local dry etching is actually carried out, unevenness which cannot be explained as a simple error remains, which is becoming a problem to be solved. As understood from the graph of FIG. 5, unevenness corresponding to the scan pitch remains. The present invention which is aimed to solve the above problem that the above unevenness remains after local dry etching has been accomplished based on the finding that this problem is caused by the above oxide film.